Recently, the use of radio frequency identification (RFID) technology to locate and track various types of items has gained increased popularity. One reason for this increase is that the costs associated with manufacturing and implementing readers and tags employing RFID technology has steadily been decreasing. In addition, RFID readers and tags have been manufactured to be ever smaller for more densely packed RFID applications.
RFID applications typically employ a single reader matched to a single antenna, as shown in FIG. 7. FIG. 7, more particularly, depicts a system diagram for two single channel, single antenna readers 700. The readers 700 are depicted as including respective reader boards 702 and capacitively coupled antenna matching circuits 704, which thus include a pair of capacitors 706 and an inductor 708. The capacitively coupled antenna matching circuit 704 enables the source impedance of the reader 700 to be matched using a pair of capacitors in parallel with the inductor 708, which operates as the antenna. The combined capacitance of the capacitors 706 in series, together with the inductor 708 provides a parallel resonant antenna.
As also shown in FIG. 7, the antenna circuit 704 forms a resonant structure that acts as a current loop. In other words, an external magnetic field 710 causes a current to be induced into the inductor 708, which circulates around the loop created by the inductor 708 and the capacitors 706. This current loop is the primary cause of antenna cross coupling when antennas are placed in close proximity to each.
More particularly, when an antenna circuit 704 is energized by a current (Is), the magnetic field 710 generated through the inductor 708 cross couples into an adjacent antenna 704. This cross-coupling causes a secondary current (Iz) to circulate in the adjacent matching circuit 704. This secondary and cross-coupled current (Iz) then, in turn, causes the magnetic field 710 to be re-radiated via the inductors 708 to the respective antenna circuits 704 that are cross-coupled to the first antenna circuit 704. This results in the undesirable effect of spreading the magnetic field 710 through the antennas of multiple readers 700. This also results in tag reads coupling across adjacent antennas 704, sometimes with multiple successive hops across multiple antennas 704, so that the relative locations of the tags with respect to the antennas 704 may be difficult or impossible to determine.